Non-deviating prisms are known. Such prisms are also known as direct view prisms. These prisms produce a fixed quantity of chromatic dispersion determined by the prism's geometry and the chromatic dispersion properties of the prism materials. One example of a non-deviating prism arrangement is the Dove prism. This type of prism is frequently used as an image rotator in a variety of opto-mechanical systems. An image passing through a dove prism will be rotated at twice the angular rate of rotation of the dove prism itself. The rotated image passes through the dove prism without angular deviation at some nominal wavelength (usually the center of the spectral band of interest) but does experience chromatic dispersion over the spectral band (e.g. the visible spectrum).
Chromatic dispersion is the separation of polychromatic light (i.e. white light) into its chromatic components (i.e. colors) by passage through a refractive element such as a prism. This is caused by the interrelationship between the index of refraction of the prism materials, the prism geometry and the frequency (or wavelength) of the light passing through the prism.
Certain kinds of variable dispersion prism arrangements are also known. One variable dispersion prism arrangement consist of a pair of rotatable prisms wedges. Unfortunately, all known variable dispersion prism arrangements also introduce a deviation (i.e. change in angle) of the nominal optical path. This deviation is typically much larger than then the total induced chromatic dispersion. It would be beneficial to have a prism arrangement which permits continuously variable chromatic dispersion without deviation of the nominal optical path. An example of how such a device would prove beneficial relates to the correction of atmospheric induced chromatic dispersion such as that encountered in terrestrial telescopic viewing of astronomical objects that are located close to the horizon.
The curvature of the Earth's atmosphere and its optical properties induces chromatic dispersion in the image of a star or planet that is proportional to the object's elevation angle above the local horizon. Due to the rotation of the earth, the elevation angle of an astronomical object is constantly changing with time (except for objects located at the celestial pole or objects viewed from the Earth's equatorial pole). Therefore, the amount of induced atmospheric dispersion also changes constantly. A commonly used prism arrangement for correcting this atmospheric dispersion is a pair of air-spaced, rotatable, wedge prisms located ahead of either the telescope objective or the telescope eyepiece. As noted above, this prism arrangement also produces substantial deviation of the nominal optical path. This angular deviation causes the image to be displaced laterally at the telescope's image plane and can even cause the image being corrected to pass out of the eyepiece's field of view. Thus, as the dispersion correction is adjusted, the telescope tracking must also be constantly adjusted to recenter the object in the field of view.
Accordingly, it is desirable to produce a variable amount of chromatic dispersion unaccompanied by deviation or at least a deviation which is very small compared to the chromatic dispersion.